The present invention relates generally to the field of semiconductor device fabrication and in particular the invention provides an improved method for electrically isolating regions of metal contact film in a semiconductor device.
A major advantage of thin-film photovoltaic (PV) modules over conventional wafer-based modules is that series interconnection of the individual cells can be accomplished using a deposited metal film. But this same film that provides electrical continuity from one cell to the next will also short out each cell unless the metal film is interrupted between each cell""s n- and p-type contacts.
Three techniques are commonly used to interrupt a thin metal film. One method is to mechanically scribe through the metal. In a second method, a strip of metal is removed with a laser. A third method is to place a mask over the silicon during metal deposition,
A pointed tool can be dragged across the surface of the metal, peeling away a narrow strip of metal. This technique works well when the underlying material is harder than the tool tip. But the method only works on planar surfaces. In a thin-film module based on polycrystalline silicon (pc-Si), surface texture is essential for light trapping, and any significant texture makes it impossible to scribe cleanly through the metal without leaving bridges across the gap or damaging the silicon.
One way of dealing with textured surfaces is to use a laser to scribe the metal by ablation. Normally, the underlying silicon is damaged in the process. This technique works in PV modules based on amorphous silicon (a-Si) because a-Si has a very high lateral resistance so that only a relatively small area of the module is impacted by the damage. But in pc-Si the lateral resistance is orders of magnitude lower, so that a single shunt ruins the entire cell.
It is also known in amorphous silicon solar cell technology to locate a light reflecting or light absorbing layer under the region of metal to be ablated to protect the underlying a-Si from damage during the ablating process.
Laser ablation is also used routinely in trimming resistor values during fabrication of analog integrated circuits. The trimming is done in inactive regions where damage to the underlying silicon does not affect the circuit""s operation. But in pc-Si solar cells, an inactive region would need to be electrically isolated from both cell polarities. This inactive region would isolate the cell on one side from the contact on the other side and vice-versa so that no current could be collected from the cell.
The other method used to interrupt a metal film is to place a mask in front of the silicon during metal deposition. The mask may be composed of taut parallel wires. The wires xe2x80x98shadexe2x80x99 small strips of the module from deposition, providing the needed interruption in the metal film. This approach works in a-Si modules because the n- and p-type contacts can be widely separated (typically 10 mm), leaving adequate room to align the wire mask. The wide contact separation is a direct result of using a transparent conductive oxide (TCO) in combination with the metal film to provide excellent lateral conductance for both contact polarities.
There are three problems with using TCO in a pc-Si module. First, there is a risk that the TCO will contaminate the silicon during high-temperature steps used to crystallise and anneal the pc-Si film. Second, it is anticipated that TCO may introduce excessive shunting where laser grooves are used to define the boundaries between cells. Third, TCO would be an expensive addition to the pc-Si fabrication sequence. Until these three issues are dealt with successfully, TCO cannot be used to increase the lateral conductance in a pc-Si module. Because the conductance of pc-Si layers are an order of magnitude smaller than TCO, the contacts in a pc-Si module must be more closely spaced (typically 1 mm), making the alignment requirements for masked deposition problematic.
U.S. Pat. Nos. 5,208,437 and 4,081,653 both disclose methods of using a laser to remove metal from a surface. U.S. Pat. No. 5,208,437 uses laser pulses of very short duration to remove the metal in small increments to avoid damaging the underlying substrate. However, this process is slow and expensive. On the other hand. U.S. Pat. No. 4,081,653 discloses a method which uses a laser to heat a region of a substrate through an overlying layer, to vaporise the substrate without melting the overlying layer to thereby cause an explosive expansion of the vaporised substrate material to remove the overlying coatings This destructive approach is not applicable to photovoltaic modules.
U.S. Pat. Nos. 4,783,421 and 4,854,974 disclose methods of forming grooves in a metal contact layer, formed over the back of an amorphous silicon solar cell array, to separate contact areas of adjacent cells. This method relies on formation of a light absorbing or light reflecting barrier laid over the amorphous silicon semi-conductor material before the metal is layered down to protect the underlying silicon while the groove is ablated in the metal.
According to a first aspect, the present invention provides a method of electrically isolating regions of a metal film located over a delicate underlying structure by parting the film along a predetermined path, the method including the steps of:
a) prior to forming the metal film, forming an inert or substantially inert layer over the underlying structure, at least in the region of a required isolation gap;
b) forming the metal film over the underlying structure and the inert layer,
c) forming a series of holes through the metal film by ablating material from the metal film along the isolation path, to separate adjacent contacts, using a pulsed long wavelength laser, the laser pulse repetition rate, scan speed and power being selected to cause incomplete ablation of the material along the isolation path, while forming overlapping adjacent melt zones, the ablated material being sufficient to enable adjacent holes to join due to surface tension while the metal in the region of the holes is molten, while leaving the inert layer unbroken in the region of the isolation path.
According to a second aspect, the present invention provides a method of applying a plurality of metal film contacts to a crystalline thin film semiconductor device, including the steps of:
a) prior to forming the metal film, forming an inert or substantially inert layer over an underlying structure, of the semiconductor device at least in the region of a required isolation gap,
b) forming the metal film over the underlying structure and the inert layer,
c) forming a series of holes through the metal film by ablating material from the metal film along the isolation path, to separate adjacent contacts, using a pulsed long wavelength laser, the laser pulse repetition rate, scan speed and power being selected to cause incomplete ablation of the material along the isolation path, while forming overlapping adjacent melt zones, the ablated material being sufficient to enable adjacent holes to join due to surface tension while the metal in the region of the holes is molten, while leaving the inert layer unbroken in the region of the isolation path.
Various embodiments of the invention provide a method in which a small amount of material is ablated from the metal film sufficient to disrupt the surface tension in the metal, without cutting through the underlying inert layer. This allows the molten metal surrounding the ablated region to draw back forming a hole, and provided that the holes are sufficiently closely spaced, the drawing back will result in the holes joining to form a continuous gap in the film. The important parameters in this process are selected according to the requirements of the particular application as follows:
1. Pulsed Long-wavelength Laser.
The power and duration of each laser pulse is preferably chosen to ablate material from the metal without cutting through the underlying inert layer. Preferably also, a wavelength is chosen which is weakly absorbed in the material under the inert layer. If possible a wavelength which is also weakly absorbed in the inert layer is preferable. Preferably also the laser is focussed to allow for variations in the height of the surface due to surface features such as texturing and lack of flatness of the underlying structure over the area of the device. For the metal/dielectric combinations tested a laser typically operated at 1064 nm and 2 kHz has been found effective. By using a high laser power with a highly defocussed beam it has been demonstrated that up to 3 mm of vertical deformation can be tolerated.
2. Thick Inert Layer.
The inert layer must be selected to withstand the heat of the laser ablating the overlying metal. The thickness will depend on the characteristics of the dielectric and the overlying metal but for Aluminium over Phosphosilicate Glass (PSG) it has been found that the PSG layer should be at least in the same order of thickness as the metal and preferably at least two times the metal thickness while thicknesses of five or ten times the metal thickness are highly preferred (typically 500-1000 nm for 100-200 nm Aluminium). In solar cell applications, the upper limit of inert layer thickness is determined by formation cost, as there are no detrimental performance effects caused by greater thickness in this application.
It has also been found effective to use organic resins such as Novolac(trademark) as the inert material in which case, the thickness of the resin would preferably be in the range of ten to twenty times the metal thickness and typical devices would have resin layers in the range of 2-4 xcexcm for aluminium layers in the range of 100-200 nm. Other dielectrics such as Silicon Dioxide or, Silicon Nitride are also effective.
3. Low Melting-point Metal.
By using a low melting point metal less laser energy is required to interrupt the metal and therefore the inert layer can be thinner. Aluminium with a melting point of 660xc2x0 C. has proven to be a suitable metal for the purpose, however in other applications metals such as Tin (232xc2x0 C.). Silver (960xc2x0 C.), Gold(1062xc2x0 C.), or Copper 1083xc2x0 C. may be used.
4. Thin Metal Layer.
The metal layer should be kept as thin as possible to achieve its purpose. Excess metal thickness requires additional energy to create a hole and accordingly requires additional inert layer thickness. For thin film solar cells metal contact films of typically 100-200 nm can be accommodated but in other applications metal layers of micron order or even of hundreds of microns may be scribed using this process.
5. Very Thin Metal Interlayer.
Tile provision of a very thin metal interlayer between the primary metal layer and the inert material can enhance the parting action of the metal by selecting an interlayer which does not adhere well to the inert layer. In the case of Novolac(trademark) resin, it has been found advantageous to interpose a layer of nickel between the resin and the primary layer which is typically aluminium. In the case of Novolac(trademark), nickel and aluminium, the aluminium layer would preferably be in the range of 5 to 20 times the nickel layer thickness and typically, a nickel layer of 10 nm would be employed with a 100 nm aluminium layer.